410 ALASKAN GLACIER STUDIES
the ice front did not advance completely across the river channel during the winter; but that it did advance all winter is proved by the breaking of the river ice, observed by the watchman at the bridge. The channel was 600 to 900 feet wide in October, 1910, and approximately 400 or 600 feet wide in June, 1911. During most of the intervening months the river was so low that a gravel bar separated the ice cliff from the river. Therefore, there was practically no loss of ice through undercutting and discharge of icebergs, and during the winter months there would be little loss from ablation. Yet the glacier did not advance completely across the river channel, as it would have done if it had been moving as much as 6^ feet a day, as was the case in the previous autumn. In fact, the rate during the winter was less than 2 feet a day.
Exactly how much the ice front did advance during the winter is not known. It was at least 200 to 400 feet and probably not much more. The river did not rise high enough to cover the gravel bar until sometime between June 6 and 9, 1911. Then the undercutting of the ice cliff by the river resulted in the beginning of rapid iceberg discharge, spoken of by people in Alaska as the "working" of the glacier. The ice cliff accordingly began to retreat and the width of river observed on June 17 had undoubtedly been increased by some retreat during the previous week. Even with this allowance it is evident that there had been a notable diminution of rate of advance during the winter of 1910-11. Judging by the incomplete data that we have, which at least give the approximate relative magnitudes of the motion, the middle of Childs Glacier was • moving at the rate of approximately 2 or 8 feet a day from 1906 to 1908, increasing to 5 or 6 feet a day in 1909, to at least SO or 40 feet a day in August, 1910, and at the height of the advance perhaps much more. The rate also decreased rapidly, being between 6 and 20 or more feet a day in October, 1910, and less than 2 feet a day in June, 1911.
Signs of the preceding summer's activity were still visible on the river bank opposite the middle of the glacier, and, pwing to a rapid rise of the river during the period of observation, the junior author was again able to witness the phenomenon of frequent and active iceberg discharge. The ice cliff was still so near that the waves occasionally rushed across the river and rose over the gravel bluff. Ice blocks of many tons in weight were left by the waves and stones of great size were hurled back among the trees, while some of the smaller shrubs were being uprooted by the waves. Several times these waves were seen to run up-stream from f to $ of a mile against the river current. Hundreds of salmon were washed out upon the bank of the river near the glacier. At the base of the ice cliff the water sometimes splashed 200 feet into the air, and air vibrations in connection with icebergs discharge shook a frame building £ of a mile from the glacier vigorously, so that the writer was awakened from sleep several times during the night. As in 1910, iceberg discharge was more frequent in night than in the daytime. Yet, on the whole, the phenomena observed in 1911 were feeble compared with those of the previous year when the maximum advance was in progress.
The diminution of movement was also shown by the inactivity of the southern third of the glacier front. Some lateral spreading of the southern margin of the glacier occurred between August, 1910, and June, 1911, but that it had ceased for some time was indicated by the presence of a narrow strip of thin ablation moraine on the ice edge and by the fact that for some distance from the margin the crevasses in the glacier had nearly healed by ablation.